Lysosomal enzymes are synthesized in human fibroblasts as precursors of higher molecular weight and slowly trimmed to the size of tissue enzymes. The enzyme responsible for the limited proteolysis of beta-hexosaminidase appears to be an acid protease, activated by thiols and inhibited by leupeptin, similar to but not identical with cathepsin B. The action of the fibroblast protease can be mimicked by trypsin and chymotrypsin. In I-Cell disease, the trimming does not occur, because the hydrolytic enzymes fail to reach lysosomes. The structure and function of the extension peptide(s) are under investigation.